1. Field of the Invention
The present invention relates to a projection lens (a zoom lens) used for projection, image taking, or the like, and an image display (projection) apparatus using the zoom lens, and an image pickup device using the zoom lens, which are suitable for, for example, a liquid crystal projector, a digital camera, and the like.
2. Related Background Art
Up to now, there have been proposed various liquid crystal projectors (image projection apparatuses) using an image display device such as a liquid crystal display device, which project an image displayed on the image display device to a screen surface.
In particular, the liquid crystal projectors have been widely used for a meeting, a presentation, or the like as an apparatus capable of projecting an image displayed on a display device of a personal computer or the like to a large screen. Recently, a small-size liquid crystal projector for home theater has been put into wide spread use. With this spread, a size of the liquid crystal display device is reducing.
In a three-plate color liquid crystal projector using three liquid crystal display devices, light from a white light source is separated into color light beams of red light, green light, and blue light by a color separation means. The separated color light beams are guided to the corresponding liquid crystal display devices. The color light beams modulated by the liquid crystal display devices are synthesized with one another by a color composition means which is composed of a dichroic prism, a polarizing plate, and the like and guided to a projection lens. Therefore, it is necessary to secure a space in which the optical elements composing the color composition means are disposed between the liquid crystal display device and the projection lens. Thus, a predetermined back focal distance is demanded for the projection lens.
When an incident angle to a color composition film provided in the dichroic prism composing the color composition means changes, a spectral transmittance changes, so, for example, uneven color occurs on a projected image. Therefore, in order to minimize the influence of angular dependence and ensure preferable pupil alignment with an illumination system, it is demanded that a projection optical system used for the color liquid crystal projector be a so-called telecentric optical system in which a pupil on the liquid crystal device (reducing) side is at infinity.
When a picture (image) produced by the color composition of three-color liquid crystal display devices is to be projected to a screen, it is necessary to superimpose respective color pixels on one another over the entire screen region. This is to prevent reductions in resolution and quality due to a double image of, for example, a character on the display device of the personal computer. Therefore, it is demanded to preferably correct a color shift (magnification chromatic aberration) caused in the projection lens in a visible light band.
In order to prevent a reduction in viewability of a projected image due to distortion of a contour portion thereof, it is demanded to sufficiently correct distortion. This is because, when a significant change in distortion is left particularly in peripheral and intermediate portions, the image quality undesirably reduces. In addition, in order to take a great deal of light fluxes from a light source means, it is demanded to use a bright lens system having a small F number.
Recently, while there are needs for increases in screen luminance, image resolution, etc., reductions in device size and weight have been strongly demanded for a projector provided with small-size panel in view of mobility. Further there has been demanded a zoom lens which can realize light and large screen projection (high luminance and wide view angle) at a shorter projection distance in a small room and easily adjust a projection screen size.
Up to now, a six-unit zoom lens has been known as the projection lens for liquid crystal projector (for example, JP 2001-108900 A). The six-unit zoom lens includes six lens units which are a lens unit having negative refracting power, a lens unit having positive refracting power, a lens unit having positive refracting power, a lens unit having negative refracting power, a lens unit having positive refracting power, and a lens unit having positive refracting power, which are disposed in order from a magnifying side (front side). Zooming is performed by suitably moving predetermined lens units of the six lens units.
In addition to the above-mentioned zoom lens, a five-unit zoom lens has been known as the projection lens for liquid crystal projector (for example, JP 2001-066499 A). The five-unit zoom lens includes five lens units which are a lens unit having negative refracting power, a lens unit having positive refracting power, a lens unit having negative refracting power, a lens unit having positive refracting power, and a lens unit having positive refracting power, which are disposed in order from the magnifying side. Zooming is performed by suitably moving predetermined lens units of the five lens units.
An aspherical lens has been used to correct aberration without an increase in the number of lenses in a zoom lens. In addition, an aspherical lens made of a plastic material has been used from the viewpoints of ease of processing and a manufacturing cost.
For example, there has been known the following zoom lens (for example, JP 2003-050352 A). The zoom lens includes three lens units which are a lens unit having negative refracting power, a lens unit having positive refracting power, and a lens unit having positive refracting power, which are disposed in order from an object side. The aspherical lens made of a plastic material is used for each of the lens units.
As compared with the case of an aspherical lens made of a glass material, the aspherical lens made of the plastic material has merits in that it is easy to manufacture, a large aspherical amount is easily obtained, and the aberration of an optical system can be easily corrected.
However, when there is a change in environment, for example, when there is a change in temperature, a refraction index of a material, a thickness thereof, a surface shape thereof, and the like significantly change. Therefore, an optical characteristic may be significantly changed to vary, for example, a focal point.
For example, when a lens made of a plastic material is used for the zoom lens for liquid crystal projector, respective lenses composing the zoom lens are heated due to a set temperature environment, heating of an illumination lamp, or the like. In particular, a refraction index of a material of a plastic lens is significantly changed by heat. Therefore, the amount of variation in focal point increases with a reduction in optical characteristic.